That depends on why you are having PGD. Ifyou are looking at chromosomes, the way it is currently being performed looks for specific sites on 7 different chromosomes. If you are trying to detct an inhertited disease carried by parents, it is test of DNA. The latter is far more sensitve and acurate. There is a new technology to be introduced soon which will be far better at detecting an abnormal number of chromosomes, but requires doing a biopsy of a blastocyst and then freezing the embryo for transfer later.